1. Field of the Invention
The present invention relates to a semiconductor device using a conductive plug for connection to a conductive structure, and in particular, it is mainly directed to a semiconductor device having a ferroelectric capacitor structure formed by sandwiching a ferroelectric film between a lower electrode and an upper electrode.
2. Description of the Related Art
In recent years, development of ferroelectric memories (FeRAMs: Ferroelectric Random Access Memories) is in progress, which retain information in a ferroelectric capacitor structure utilizing polarization inversion of a ferroelectric. The ferroelectric memories are non-volatile memories which do not lose information retained therein even when the power supply is cut off, and attracting attention particularly because they promise realization of high degree of integration, high speed operation, high durability, and low power consumption.
Ferroelectric capacitors have characteristics which deteriorate easily by hydrogen gas and moisture from the outside. Further, their characteristics easily deteriorate also by etching processing or the like. Accordingly, recovery of capacitor characteristics by means of high-temperature annealing is essential. Then, as materials for an upper electrode and a lower electrode constituting a ferroelectric capacitor structure, noble metals and noble metal oxides which endure the high-temperature annealing are widely used.    [Patent document 1] Japanese Patent Application Laid-open No. 2001-230382    [Patent document 2] Japanese Patent Application Laid-open No. 2003-110095    [Patent document 3] Japanese Patent Application Laid-open No. 2004-303995
In FeRAMs, noble metals such as platinum (Pt) and layered films of a noble metal oxide and a noble metal are widely used for the upper electrode of the ferroelectric capacitor structure. However, noble metals such as Pt have catalytic activity, which operates to activate oxygen existing in a conductive oxide film constituting the upper electrode and an insulating film in which the ferroelectric capacitor structure is embedded or oxygen entering from the outside. On the ferroelectric capacitor structure, a conductive plug constituted of a material such as tungsten (W) is formed via a conductive base film (glue film) of TiN/Ti or the like, where activated oxygen as described above oxidizes the glue film and the conductive plug. As a result, contact resistance increases and causes a problem such that a sufficient voltage will not be applied to the ferroelectric capacitor structure. Activated oxygen generated by catalytic activity of a noble metal has an ability to oxidize the glue film and the conductive plug even at room temperature.
Also, even when no noble metal layer of Pt or the like, in other words, no catalyst exists in the upper electrode, oxygen is activated by high-temperature recovery annealing, which is performed on the ferroelectric film while growing W by a CVD method when forming the conductive plug or after a via hole is formed. This activated oxygen oxidizes the glue film and the conductive plug.